- Added support for configurable 'allowBlank' as proposed by tayfun.ozis.erikan: http://www.sencha.com/forum/showthread.php?134345-Ext.ux.form.field.DateTime/page2
 - Added support for 'isValid()' as proposed by espeak:  http://www.sencha.com/forum/showthread.php?134345-Ext.ux.form.field.DateTime/page2